The overall goal of this study is to determine the mechanisms of tumor necrosis factor-alpha's (TNF's) anti-leukemic activity in vivo. TNF, a macrophage product, kills tumor cells in vitro and in vivo and is cytotoxic to virus-infected cells with inhibition of viral replication observed in some systems. In addition to anti-tumor/virus activities, TNF has been shown to be involved in cell regulatory functions including activation of the immune response and effects on hematopoietic progenitors in vivo. We have demonstrated that TNF significantly suppresses late-stage Friend virus (FV) leukemic erythroid progenitors (CFU-E) in vivo. Suppression of CFU-E leads to permanent regression when animals are treated with either TNF or TNF plus interferon-gamma (IFN-gamma). TNF also significantly suppresses CFU-E in normal animals resulting in anemia which is reversed by erythropoietin (EPO). TNF has no direct effect in vitro on normal, but is cytotoxic to virus infected cells. T and NK cells are not involved in TNF's hematopoietic effects, but L3T4+ cells appear to be required for TNF induced regression. Therefore, we propose to examine whether TNF's anti- erythroleukemic activity is the result of TNF's suppressive effect on normal erythroid progenitors or the result of a specific immunologic or anti-virus response. The aims of this study include: 1) to determine the mechanisms of TNF's suppressive effect in vivo on late-stage erythropoiesis by evaluating whether a) erythroid cells lose or shed their TNF receptor as they differentiate, b) TNF acts on erythroid cells by affecting their interaction with EPO, and c) TNF acts indirectly through induction of another cytokine; 2) to examine the interaction between TNF and L3T4+ cells in the regression of erythroleukemia by up-regulation of IL-2 and TNF receptors, enhanced cytokine production, increased proliferation and enhanced class II antigen expression; 3) to determine whether TNF directly interferes with the replication of FV in macrophages and erythroblasts by changes in reverse transcriptase levels, viral protein expression, and the presence of viral-specific RNA; 4) to determine whether TNF induces INF in FV infected macrophages with a correlation to an anti-viral effect. These studies form the basis for analyzing the mechanisms of these diverse effects in vitro and in vivo with possible application to the therapeutic use of TNF or related cytokines for hematopoietic malignancies and diseases with a viral etiology.